From my experience with US naval vessels I would have to reccomend looking for a Garloc gasket. They make gasket/sealing material for nearly every application imaginable. Working with some of their thicker "cut to size" materials could be a pain if you don''t have the appropriate equipment (or access to it) but the results are always favorable. A simple web search should give you atleast a handful of places to order garloc gasket material from as well as what type would fit your purpose.